Salt bath for case-hardening



Patented a. 10, i 939 1 SALT BATH FOR GASE-HARDENING Ernest A. Walen, Springfield, Mass, assignor The Heatbath Corporation, West Springfield,

Mass, a corporation of Massachusetts N0 Drawing. Application April 1, 1938,

Serial No. 199,533

\ 1 Claim.

The action of my improved salt bath is based principally upon the presence therein of a cyanide modified by the influence of one or 'more suitable accelerators, the latter being chosen with a view not only of speeding up the rate of cementation, but also for producing only such residue products of the reaction or double reactions taking place during the case hardening process as will be readily soluble in water.

An objectionable characteristic of cyanide salt baths either with or without accelerators as heretofore used is that when the steel articles are removed therefrom after the carburizing treatment, special cleaning processes must be resorted.

to for removinginsoluble end products adhering to the articles so treated, due to the fact that said end products are not soluble. In the use of my improved bath composition hereinafter described, the articles after being treated may be thoroughly cleaned simply by washing in water which is a considerable saving in expense over old methods.

Another advantage of my improved bath composition is that it may be used at relatively high working temperatures without serious decomposition or breaking down of the active carburizing agents therein. Temperatures of 1350 F. to 1750 F. are safe working limits since on test my bath 36 composition has remained stable at a temperature of 1800 F. for as long as one hundred hours. 4

Furthermore, my bath composition is so constituted that sludge of residue products is not formed at the bottom of the pot which would 40 tend to insulate the transfer'of heat to the bath when in use. i

In the continuous use of any bath with work being repeatedly immersed therein and withdrawn therefrom a gradual depletion thereof 45 takes place due to the so called drag-out or portion of the bath adhering to the withdrawn work. In the replenishment of my bath a composition of the same original proportions is added since neither the drag-out loss nor the reac- 50 tions of working the bath changes the original composition. A further advantage of my bath is that its combustibility is very low, there being only a very slow oxidizing thereof at the surface in contact with air. For this reason my bath does not diminish in its active carburizing 7 strength over long periods, and it has been found in practice that the replenishment of the dragout loss even when infrequently made will be entirely suflicient to keep the bath in its full working carburizing strength. 5 As has been stated, the principal ingredient of my bath composition is a cyanide and preferably sodium cyanide (NaCN) because it gives a soluble characteristic to the composition and products thereof and is also more stable than other 10 cyanide compounds which otherwise might be used. To the sodium cyanide I add as the principal accelerator, strontium chloride (SrCl-z) which in combination with the sodium cyanide carburizes the steel. While the sodium cyanide predominates in quantity in the composition the accelerator strontium chloride is also preferably added in a liberal proportion so that unless restrained the initial activity of said reaction might, be faster than desired. Accordingly I add to 20 the aforesaid (Na-300a) as a stabilizer which acts to level out the intensity of said reaction, prolong the life of the bath, and make its action more uniform.

This sodium carbonate is also chosen because 25.

of its soluble character since it is important in. my bath that all end products of the reacting bath shall be soluble in water for easy cleaning by washing. An equivalent of the sodium carbonate for my composition is strantium carbon- 31) ate, and this latter might be used except for the fact of its much higher cost at the present time.

The foregoing compounds are the chief working parts of my improved bath composition and are fused together in the approximate propor- 35 tions by weight as follows:

Parts Sodium cyanide (NaCN) 4 Strontium chloride (SrClz) 2 Sodium carbonate (Na2CO3) 40 and/or 1 Strontium carbonate (SICOs) However, a bath of the above composition for case hardening would give a case too high in carbon and preferably should be diluted for best results, The further compounds added as determined by low cost, considerations of solubility, low fusing temperatures, stability, etc. are preferably sodium chloride (NaCl), potassium chloride (KCl), and sodium fluoride (NaF). The last named fills the function of a'cleaner for foreign matter such as scale (iron oxide) that may be introduced into the bath on the work being treated. It tends to dissolve these things and keep them in solution rather than peringredients sodium carbonate mitting them to accumulate in the bottom of the pot as an objectionable and insulating sludge. Sodium fluoride ingredient is not so important as the others and may in some cases be omitted.

-A typical example in approximate percentages of my preferred 'and complete composition is given as follows:

' Per cent Sodium cyanide (NaCN) -f 40 Strontium chloride (SrC12).. 20 Sodium carbonate (NazCO3) and/or 10 Strontium carbonate (SrCOa) Sodium chloride (NaCl) 20.45 Potassium chloride (KCl) 5 Sodium fluoride *(NaF); 4.55

The foregoing bath composition fuses at approximately 1075" F. and its normal or most commonly used working temperature is about 1550 F. As heretofore stated, however, it can be successfully used at temperatures up to 1750 F. or 1800" F. without breakdown. In carburizing steels for case hardening its rate of penetration is from fifty to one hundred per cent faster than the ordinary and commonly used cyanide salt baths, the depth of penetration depending upon the duration of the steel immersion therein. The bath is prepared for the market in the usual way by fusing together the ingredients in the proportions above stated and breaking up the solidified product into condition suitable for shipment.

What I claim is:

A fusible salt bath composition for carburizing steels of relatively low carbon content comprising in approximate proportions by weight the following:

Per cent Sodium -cyanide 40 Strontium chloride 20 Sodium carbonate 10 Sodium chloride 20.45 Potassium chloride 5 Sodium fluoride 4.55

ERNEST A. WALEN. 

